In this tab you can pre-define Templates to pre-define Artificial Tasks. You simply have to put a Template's 'Name', 'Description' and individual Task names on the 'Task List' and press the 'Save' button. This will result in the group of tasks being arranged in an independent micro-structure, which later on can be re-used as a template in your Gantt.
In order to achieve a pre-configured arrangement of that sort, you need to use the relative dates column. Relative, because the numbers you put there are counted since the Parent Task (if there is any). Once saved, the template will appear in your Task Templates list in the '+Task' menu and you will be free to fit it in your structure in any desired way with pre-defined dependency rules.
This is very handy feature when you manage complex projects with reusable sets of tasks. For example, once you have some 'Marketing' stories in your project along with 'Development' stories which you re-use every Sprint or so, uoy can create specific sets of Task Templates to add to your Gantt chart every ongoing Sprint / Version or any repeatable time period.